Datasheet
LTC4268-1
18
42681fc
During the inrush event as C1 is being charged, a large
amount of power is dissipated in the MOSFET. The
LTC4268-1 is designed to accept this load and is thermally
protected to avoid damage to the onboard power MOSFET.
If a thermal overload does occur, the power MOSFET turns
off, allowing the die to cool. Once the die has returned to
a safe temperature, the LTC4268-1 automatically switches
to I
LIMIT_LOW
, and load capacitor C1 charging resumes.
The LTC4268-1 has the option of disabling the normal
operating input current limit, I
LIMIT_HIGH
, for custom
high power PoE applications. To disable the current limit,
connect I
LIM_EN
to V
PORTN
. To protect the LTC4268-1
from damage when the normal current limit is disabled, a
safeguard current limit, I
LIMIT_DISA
keeps the current below
destructive levels, typically 1.4A. Note that continuous
operation at or near the safeguard current limit will rapidly
overheat the LTC4268-1, engaging the thermal protection
circuit. For normal operations, float the I
LIM_EN
pin. The
LTC4268-1 maintains the I
LIMIT_LOW
inrush current limit
for charging the load capacitor regardless of the state of
I
LIM_EN
. The operation of the I
LIM_EN
pin is summarized
in Table 3.
Table 3. Summary of IEEE 802.3af Power Classifications and
LTC4268-1 R
CLASS
Resistor Selection
STATE OF I
LIM_EN
INRUSH CURRENT
LIMIT
OPERATING INPUT
CURRENT LIMIT
Floating I
LIMIT_LOW
I
LIMT_HIGH
Tied to V
PORTN
I
LIMIT_LOW
I
LIMIT_DISA
POWER GOOD
The LTC4268-1 includes complementary power good
outputs (Figure 8) to simplify connection to any DC/DC
converter. Power Good is asserted at the end of the inrush
event when load capacitor C1 is fully charged and the
DC/DC converter can safely begin operation. The power
good signal stays active during normal operation and is
de-asserted at power off when the port drops below the
UVLO threshold or in the case of a thermal overload event.
For PD designs that use a large load capacitor and also
applicaTions inForMaTion
42681 F08
BOLD LINE INDICATES HIGH CURRENT PATH
PWRGD
POWER
NOT
GOOD
INRUSH COMPLETE
AND NOT IN THERMAL SHUTDOWN
V
PORT
< UVLO OFF
OR THERMAL SHUTDOWN
POWER
GOOD
29
PWRGD
LTC4268-1
30
V
NEG
28
V
NEG
26
V
PORTN
7
V
NEG
27V
PORTN
6
V
PORTN
REF
UVLO
THERMAL SD
5
CONTROL
CIRCUIT
Figure 8. LTC4268-1 Power Good Functional and State Diagram